1. Field of the Invention
The present invention relates to a coating solution for forming a silica film.
2. Background Art
Along with recent requirements for miniaturization of semiconductor devices and high-speed processing of electric signals, reduction in the wiring delay is demanded. From the standpoint of decreasing the wiring resistance, there is a movement of shifting the wiring material from conventional aluminum to copper. Also, from the standpoint of decreasing the wiring capacity, reduction in the dielectric constant of interlayer insulating film is demanded.
With respect to the interlayer insulating film material having a low dielectric constant, various materials have been proposed. For example, there are known CVD materials such as P-TEOS (plasma tetraethoxysilane), SiOF and SiOC, and coating materials (SOD materials) including inorganic SOG (silicon on glass) materials such as HSQ (hydrogensilsesquioxane) and porous HSQ; organic SOG materials such as MSQ (methylsilsesquioxane); porous MSQ; and organic polymer-based materials such as polyarylene ether (PAE). These interlayer insulating film materials have achieved a dielectric constant on the order of 1.8 to 4.1. In particular, low dielectric constant materials (low-K materials) having a dielectric constant of less than 4 are drawing attention as a material suitable for multilayer interconnection techniques using a fine copper wiring.
As for the technique of more reducing the dielectric constant of interlayer insulating film, for example, a method of rendering the film porous has been proposed. However, when the film becomes porous, the hygroscopicity of film is elevated and the dielectric constant of film tends to increase in aging, giving rise to poor aging stability. Furthermore, as the film becomes porous, the mechanical strength of film is deteriorated and this causes a problem such as generation of cracking in the CMP (chemical mechanical polishing) process or the like. As for the material capable of achieving reduction in the dielectric constant, a silica film-forming material having added thereto a surfactant having directing property and an ammonium salt has been proposed (see, JP-A-2002-26003 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). However, this coating solution is not acid-controlled and therefore, improvement in storage stability of the coating solution is being demanded. As for the material capable of realizing reduction in the dielectric constant and increase in the mechanical strength, a silica film-forming material obtained by adding an ammonium salt having orienting property to a coating solution using a tetraalkoxysilane as the basic ingredient has been proposed (see, Takamaro Kikkawa, et al., Nikkei Microdevices, pp. 123–132 (February, 2003)). However, when only a tetraalkoxysilane is used as the basic ingredient, the obtained film tends to have high water adsorptivity and this causes a problem that the dielectric constant of film is elevated in aging. Furthermore, the halogen ion used as the counter ion of the ammonium salt, such as chloride ion and bromide ion, may corrode a metal and this disadvantageously disturbs the application to semiconductor devices.
Under these circumstances, a silica film-forming material having a low dielectric constant and giving a film of less undergoing change in aging is being demanded. Also, a silica film-forming material having good storage stability in addition to those properties is being demanded.